Toward understanding transverse relaxation in human brain through its field dependence.

Abstract

Apparent transverse-relaxation rate constants (R₂⁺ = 1/T₂⁺) were measured in various regions of the healthy human brain using a multiecho adiabatic spin-echo sequence at five different magnetic fields, 1.5, 1.9, 3, 4.7, and 7 T. The R₂⁺ values showed a clear dependence on magnetic field strength (B(0) ). The regional distribution of the R ₂⁺ was well explained by the sum of three components: (1) regional nonhemin iron concentration ([Fe]), (2) regional macromolecular mass fraction (f(M) ), and (3) a region-independent factor. Accordingly, R₂⁺ = α[Fe] + βf(M) + γ, where coefficients α, β, and γ were experimentally determined at each magnetic field by a least square fitting method using multiple regression analysis. Although the coefficient α linearly increased with B(0) , β showed a quadratic dependence on top of a field-independent component. The coefficient γ also increased slightly with B(0) on top of a field-independent component. The linear dependence of α on B(0) was consistent with that observed for the transverse-relaxation rate of water protons in ferritin solutions as found previously by others. The quadratic dependence of β on B(0) was accounted for by isochronous and anisochronous exchange mechanisms using intrinsic-relaxation parameters obtained from the literature.

T2† maps of human brain obtained with the MASE sequence at (a) 1.5T, (b) 1.9T, (c) 3T, (d) 4.7T, and (e) 7T. TR = 4000ms, TE = 26, 52, 78, 104, 130, and 156ms. A data matrix of 256 × 128 was obtained for the FOV of 25.6 × 25.6 cm with the slice thickness of 2.5 mm. (a), (c), and (e) were obtained with the same subject, (b) and (d) with another subject. The six regions considered for further evaluation in this study are indicated in (d).

Field dependent change in (a) coefficient α, (b) coefficient β, and (c) coefficient γ. Solid line shows the result of linear fittings for α and γ, and of quadratic fitting for β, assuming y = ax2 + b, where a, and b are constant values.

Simulated R2MM values due to macromolecular contribution using with parameters listed in as a function of the static field strength (solid line) compared with βfM terms (closed circles) at 1.5, 1.9, 3, 4.7, and 7T in six brain regions. Error bars indicate standard errors in βfM.